Brillouin scattering is the inelastic scattering of light in a medium when it encounters a time-varying density of refractive index within the medium in which it is travelling. The scattered light undergoes a frequency shift which is detected and provides information about the material. The density fluctuations can be caused by temperature gradients, magnetic fields or, most commonly, acoustic waves.
The electric field of an intense laser beam can itself produce acoustic vibrations in a medium. The beam may then undergo Brillouin scattering from these vibrations (usually in the opposing direction of the incoming beam) – this phenomenon is known as Stimulated Brillouin Scattering (SBS.) This can be thought of as a 4-photon/phonon mixing process (two to set-up a 3-D diffraction grating and an incident and a diffracted photon from the grating).
When used with sound waves, the Brillouin scattering effect can be used to probe and measure the elastic behaviour of small particles, thin films or the surface characteristics of bulk materials. Brillouin scattering can measure important characteristics such as annealing, residual stress and viscosity.
The laser used must have a narrow linewidth, long coherence length and show stable wavelength over time to ensure the interferometry results are a result of faint Brillouin scattering rather than laser effects. The torus lasers are ideal for this application due to the single longitudinal mode with an active locking mechanism to eliminate mode hop and reduce wavelength drift to <2pm over 20°C.